In photosynthesis, the plant uses light energy to turn CO₂ and water into sugars. These sugars build leaves and roots. The work is done by the pigments chlorophyll a and b, which absorb blue and red light best. Other pigments, such as carotenoids, help catch light in between.
That is why grow lights focus on the 400 to 700 nm range, called PAR. More about PAR in our article on light measurements.
Grass also senses the colour of light through receptors called phytochromes. They respond to red light (around 660 nm) and far red light (around 730 nm).
At sunrise and sunset, natural light contains more far red and less red. Grass reads this as a signal. Under neighbouring plants or in shade, it sees the same shift and reacts by stretching up to find more light. This is called the shade avoidance response. Research shows how strongly the light spectrum shapes plant growth, structure and even defence (De Wit et al., 2016).
The length of the day, called the photoperiod, also steers growth. It affects how fast leaves expand and how much dry matter the plant builds. Light in the right amount, at the right time and in the right colour gives the strongest result.
